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    <title>Document</title>
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<body>
    <script>
        /**
         * var arr = [1,2,4,5];
         * var index = 2;
         * var num = 3;
         * 
         * ==> [1,2,3,4,5]
         * 
         */

        var arr = [2, 3, 4, 5];
        var num = 100;
        var index = 2;
        for (var i = arr.length - 1; i >= index; i--) {
            arr[i + 1] = arr[i];
        }
        
        arr[index] = num;
        console.log(arr);
    </script>
</body>

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